Avoid potentially expensive inspect call in router

The attached patch defers the creation of a routing error message in ActionDispatch::Routing::RouteSet::Generator#generate which invoke inspect on the argument of url_for (or any routing helper method actually).

This call might be expensive, especially when the argument happens to be a database record with blob fields, and is currently executed even if the route is valid.

For example, when using SlimScrooge with the following view:

<%= link_to foo.name, foo %>

SELECTed fields are expected to be id and name, but happen to be all fields from the table.

The patch applies on HEAD and fixes the above-mentioned issue, all tests are passing.